1. Field Of The Invention
This invention relates to servicing of process flowlines, and more particularly to methods for forming temporary plugs in process flowlines so that servicing operations on the flowlines can be conducted. A specific embodiment of the invention relates to a method for forming a temporary plug in one of a plurality of well production flowlines extending from a plurality of wells to a common processing unit. For example, flowlines from a plurality of offshore wells typically extend to a platform-mounted separator, and occasionally one of the flowlines requires servicing, such as replacement of a leaking valve or repair of a section of the flowline It is desirable to keep the other wells on production when a particular well flowline needs servicing, and the present invention provides an improved method of accomplishing that goal.
2 The Prior Art
It is common practice to isolate a section of a process flowline for purposes of servicing the flowline. This is preferably done by closing valves on the flowline on both sides of the point requiring service. However, very often servicing is required where valves are not available for isolating the pertinent part of the flowline. In such cases, temporary plugs have been formed by injecting water into the flowline at a point to be plugged, and then freezing the water in place. This technique has been used with some success in the past, but is subject to certain problems In areas of high ambient temperature, an excessive amount of cooling is required to form the plug. For example, many hours or even days are required to form a plug of frozen water in large diameter lines at ambient temperatures of 20.degree. C. or more. Also, the expansion properties of water upon freezing can cause damage to the flowline.
U.S. Pat. Nos. 3,827,282; 4,112,706; 4,220,012; 4,370,862 and 4,416,118 to Brister all describe means for freezing a water plug in a fluid transmission line.
U.S. Pat. No. 3,623,337 to Tremont describes an apparatus for freezing liquid in a section of pipe.
U.S. Pat. No. 4,383,783 to Kraka et al., and U.S. Pat. No. 4,379,722 to Scott disclose use of a moveable gel plug during construction of an offshore pipeline.
U.S. Pat. No. 4,739,799 to Carney et al. discloses a frangible and soluble plug for temporarily blocking a sewer pipe.
The prior art discussed above has not dealt with the problem of repairing a process flowline in high ambient temperature conditions with a plug formed of a material having a freezing point higher than the freezing point of water.